Characterizing the performance of human leg external force control

Abstract

Our legs act as our primary contact with the surrounding environment, generating external forces that enable agile motion. To be agile, the nervous system has to control both the magnitude of the force that the feet apply to the ground and the point of application of this force. The purpose of this study was to characterize the performance of the healthy human neuromechanical system in controlling the force-magnitude and position of an externally applied force. To accomplish this, we built an apparatus that immobilized participants but allowed them to exert variable but controlled external forces with a single leg onto a ground embedded force plate. We provided real-time visual feedback of either the leg force-magnitude or force-position that participants were exerting against the force platform and instructed participants to best match their real-time signal to prescribed target step functions. We tested target step functions of a range of sizes and quantified the responsiveness and accuracy of the control. For the control of force-magnitude and for intermediate step sizes of 0.45 bodyweights, we found a bandwidth of 1.8 ± 0.5 Hz, a steady-state error of 2.6 ± 0.9%, and a steady-state variability of 2.7 ± 0.9%. We found similar control performance in terms of responsiveness and accuracy across step sizes and between force-magnitude and position control. Increases in responsiveness correlated with reductions in other measures of control performance, such as a greater magnitude of overshooting. We modelled the observed control performance and found that a second-order model was a good predictor of external leg force control. We discuss how benchmarking force control performance in young healthy humans aids in understanding differences in agility between humans, between humans and other animals, and between humans and engineered systems.

Human force control may trade-off force error with central tendency and recency biases

“…RMS errors indeed roughly increased with the force magnitude in these cases. Directionality of the error during these sub-trials was also consistent with previous studies: we predict and measure negative errors, and previous studies also usually measured negative force tracking errors (e.g., directly reported in Todorov (2002 ), could be inferred from figures in ( Kudzia et al, 2022 )). However, what had been usually not measured and reported in previous studies is that RMS force error during zero force sub-trials increases again (figure 12A), and that people tend to make a positive error for this range.…”

Neuromuscular fatigue reduces responsiveness when controlling leg external forces

“…To characterize human leg force control, we tested the step response of participants as they selectively controlled external leg force. We used a custom apparatus (Figure 1) that we had previously built [1]. The apparatus consisted of a ground-embedded force plate (Bertec Corporation, Ohio, USA) that participants stood on, constrained in both the vertical and horizontal directions.…”

“…Although control of force-position in the medial-lateral and anterior-posterior directions (i.e., center of pressure control) and control of all three orthogonal force magnitude components of the external force vector is important, our work here focused only on the control of vertical force magnitude. In our prior work, we characterized the control of different step sizes of mediallateral and anterior-posterior force positions, as well as the control of a range of sub-maximal vertical force magnitudes and found negligible differences in control characteristics for these different components of the external force vector [1]. By focusing on only the vertical component of force, we believe our findings will represent the range of control characteristics for all aspects of controlling the external force vector at submaximal forces and any control changes resulting from fatigue.…”

“…After placing the EMG sensors, we fitted participants into the experimental apparatus. When fitting participants, we adjusted the apparatus such that participants maintained an approximately 15degree knee flexion angle in their right leg, a posture similar to that assumed by a runner at the start of the stance phase [1], [21]. In this fitting process, we moved components of the apparatus to push down on the participant's shoulders and up on their arms by manually rotating four scissor jacks (Figure 1).…”

“…The resulting reaction force from the environment, a vector quantity, accelerates our bodies. Here we consider the rapid and accurate control of the external force vector to be agility [1]. Examples of legged agility include accelerating from a standstill, jumping high and far over obstacles, and quickly changing movement direction, all of which require effective control of our leg's external forces.…”